In a node for constructing an optical network, route switching through OADM (Optical Add Drop Multiplexing) is performed, and a WDM (Wavelength Division Multiplexing) transmission in which signal light having multiple wavelengths different from each other is multiplexed is performed.
In the OADM, particularly, an arbitrary wavelength channel is added from each port of nodes, and colorless OADM in which an arbitrary wavelength channel is dropped is realized.
As a conventional technology, a technology for expanding the OADM function is proposed upon request. A technology for removing feet of optical spectra is further proposed.    Japanese Laid-open Patent Publication No. 2006-87062    Japanese Laid-open Patent Publication No. 2009-212584
As described above, when a wavelength channel is added or dropped to perform WDM transmission by using the colorless OADM, efficiency of wavelength routing is improved.
However, when wavelength channels having wavelengths different from each other are wavelength-multiplexed, there is a problem that crosstalk between wavelength channels occurs. Particularly, as a transmission data rate is larger, a bandwidth of an optical spectrum is wider. Therefore, an influence of crosstalk between adjacent wavelength channels is more exerted to degrade transmission quality.